The present invention relates generally to spread spectrum communications and Global Positioning Systems and, in particular embodiments, to systems, processes and devices which improve the performance of Global Positioning Systems in the presence of jamming signals.
It is likely that the first navigational system used by man was the sun. It rose in the east and set in the west and as long as travel occurred in daylight hours general direction could be obtained from the sun""s position in the sky. With the advent of commerce on the seas it became necessary to ascertain direction at night, and so stellar navigation was born. In order to increase their accuracy, stellar and solar navigation techniques were improved and augmented through the use of maps, charts, and instruments such as the astrolabe and compass. Even augmented by such instruments, stellar and solar navigation was error prone and getting from point A to point B was still, to some extent, a matter of trial and error.
With the advent of radio, and particularly powerful commercial radio stations land based radio direction finding (RDF) came into being. The principle behind RDF is relatively simple. A navigator can tune to a radio station using a directional antenna to find the directional bearing of the radio station. The navigator can then could tune to a second radio station and find the bearing of that station. By knowing the bearing and map location of both stations the navigator""s position can be calculated.
Continuing advances in long distance air travel necessitated the ability to guide aircraft accurately. RDF was used to satisfy this requirement and land based beacons were established for the purpose of navigation. These beacons quickly became indispensable to all aviation and to ships as well.
The Global Positioning System (GPS) is also based on radio navigation, a difference being that the beacons are no longer stationary and are no longer land based. The GPS system is a satellite based navigation system having a network of 24 satellites, plus on orbit spares, orbiting the earth 11,000 nautical miles in space, in six evenly distributed orbits. Each satellite orbits the earth every twelve hours.
A prime function of the GPS satellites is to serve as a clock. Each satellite derives its signals from an on board 10.23 MHz Cesium atomic clock. Each satellite transmits a spread spectrum signal with its own individual pseudo noise (PN) code. By transmitting several signals over the same spectrum using distinctly different PN coding sequences the satellites may share the same bandwidth without interfering with each other. The code used in the GPS system is 1023 bits long and is sent at a rate of 1.023 megabits per second, yielding a time mark, sometimes called a xe2x80x9cchipxe2x80x9d approximately once every micro-second. The sequence repeats once every millisecond and is called the course acquisition code (C/A code). Every 20th cycle the code can change phase and is used to encode a 1500 bit long message which contains an xe2x80x9calmanacxe2x80x9d containing data on all the other satellites.
There are 32 PN codes designated by the GPS authority. Twenty four of them belong to current satellites in orbit, the 25th PN code is designated as not being assigned to any satellite. The remaining codes are spare codes which may be used in new satellites to replace old or failing units. A GPS receiver may, using the different PN sequences, search the signal spectrum looking for a match. If the GPS receiver finds a match, then it has identified the satellite which has generated the signal.
Ground based GPS receivers may use a variant of radio direction finding (RDF) methodology, called triangulation, in order to determine the position of the ground based GPS receiver. The GPS position determination is different from the RDF technology in that the radio beacons are no longer stationary, they are satellites moving through space at a speed of about 1.8 miles per second as they orbit the earth. By being space based, the GPS system can be used to establish the position of virtually any point on Earth using methods such as triangulation.
The triangulation method depends on the GPS receiving unit obtaining a time signal from a satellite. By knowing the actual time and comparing it to the time that is received from the satellite the receiver, the distance to the satellite can be calculated. If, for example, the GPS satellite is 12,000 miles from the receiver then the receiver must be somewhere on the location sphere defined by the radius of 12,000 miles from that satellite. If the GPS receiver then ascertains the position of a second satellite it can calculate the receiver""s location based on a location sphere around the second satellite. The two sphere""s intersect and form a circle, and so the GPS receiver must be located somewhere within that location circle. By ascertaining the distance to a third satellite the GPS receiver can project a location sphere around the third satellite. The third satellite""s location sphere will then intersect the location circle produced by the intersection of the location spheres of the first two satellites at just two points. By determining the location sphere of one more satellite, whose location sphere will intersect one of the two possible location points, the precise position of the GPS receiver is determined. As a consequence, the exact time may also be determined, because there is only one time offset that can account for the positions of all the satellites. The triangulation method may yield positional accuracy on the order of 30 meters, however the accuracy of GPS position determination may be degraded due to signal strength and multipath reflections.
As many as 11 satellites may be received by a GPS receiver at one time. In certain environments such as a canyon, some satellites may be blocked out, and the GPS position determining system may depend for position information on satellites that have weaker signal strengths, such as satellites near the horizon. In other cases overhead foliage may reduce the signal strength that is received by the GPS receiver unit. In either case the signal strength may be reduced.
There are multiple ways of using radio spectrum to communicate. For example in frequency division multiple access (FDMA) systems, the frequency band is divided into a series of frequency slots and different transmitters are allotted different frequency slots.
In time division multiple access (TDMA) systems, the time that each transmitter may broadcast is limited to a time slot, such that transmitters transmit their messages one after another, only transmitting during their allotted period. With TDMA, the frequency upon which each transmitter transmits may be a constant frequency or may be continuously changing (frequency hopping).
A third way of allotting the radio spectrum to multiple users is through the use of code division multiple access (CDMA) also known as spread spectrum. In CDMA all the users transmit on the same frequency band all of the time. Each user has a dedicated code that is used to separate that user""s transmission from all others. This code is commonly referred to as a spreading code, because it spreads the information across the band. The code is also commonly referred to as a Pseudo Noise or PN code. In a CDMA transmission, each bit of transmitted data is replaced by that particular user""s spreading code if the data to be transmitted is a xe2x80x9c1xe2x80x9d, and is replaced by the inverse of the spreading, code if the data to be transmitted is xe2x80x9c0xe2x80x9d.
To decode the transmission at the receiver it is necessary to xe2x80x9cdespreadxe2x80x9d the code. The despreading process takes the incoming signal and multiplies it by the spreading code and sums the result. This process is commonly known as correlation, and it is commonly said that the signal is correlated with the PN code. The result of the despreading process is that the original data may be separated from all the other transmissions, and the original signal may be recovered. A property of the PN codes that are used in CDMA systems is that the presence of one spread spectrum code does not change the result of the decoding of another code. The property that one code does not interfere with the presence of another code is often referred to as orthogonality, and codes which possess this property are said to be orthogonal.
The process of extracting data from a spread spectrum signal is commonly known by many terms such as correlating, decoding, and despreading. Those terms will be used interchangeably herein.
The codes used by a spread spectrum system are commonly referred to by a variety of terms including, but not limited to, PN (Pseudo Noise) codes, PRC (Pseudo Random Codes), spreading code, despreading code, and orthogonal code. Those terms will also be used interchangeably herein.
It is because CDMA spreads the data across the broadcast spectrum that CDMA is often referred to as spread spectrum. Spread spectrum has a number of benefits. One benefit being that because the data transmitted is spread across the spectrum, spread spectrum can tolerate interference better than some other protocols. Another benefit is that messages can be transmitted with low power and still be decoded, and yet another benefit is that several signals can be received simultaneously with one receiver.
The Global Positioning System (GPS) uses spread spectrum technology to convey its data to ground units. The use of spread spectrum is especially advantageous in the GPS systems. Spread spectrum technology enables GPS receivers to operate on a single frequency, thus saving the additional electronics that would be needed to switch and tune other bands if multiple frequencies were used. Spread Spectrum also minimize power consumption requirements. GPS transmitters for example require 50 watts or less and tolerate substantial interference.
Although the GPS system is available widely, there are conditions which can degrade its performance or block its use. Improvements in the reception of GPS signals are constantly being sought. There is a need for greater sensitivity in receiving GPS signals to improve the performance of ground based receivers.
Accordingly, preferred embodiments of the present invention are directed to GPS receivers, which employ spread spectrum technology, and systems which employ GPS receivers.
To receive the data a spread spectrum receiver must determine that a PN code is present within the spread spectrum signal. To determine that a PN code is present in a received signal, the received spread spectrum signal must be compared, bit by bit, to the PN code. This process of comparing a signal bit by bit to a PN code is commonly known as correlation. Typically correlators within a GPS unit compare known GPS codes to the received spread spectrum signal to determine the presence of a given GPS signal. The GPS system will look for high outputs from the correlators as an indication that the signal containing a particular PN code (i.e. a signal from a particular GPS satellite) has been found.
Jamming interference from a variety of sources can disrupt the process of finding and decoding GPS signals. When the process of finding and decoding GPS signals is disrupted, errors can occur. These errors may build up if jamming continues. As a result of these, jamming induced, built up errors significant errors in position may occur. These jamming induced errors may persist for long periods of time, up to hours, even after the jamming has ceased.
Several of the preferred embodiments of the disclosure seek to prevent errors in GPS systems related to jamming. These embodiments detect indications that a jamming signal is present and then apply countermeasures to remediate its effects.
There are several methods of detecting that a jamming signal may be present. By applying these methods, alone or together, indications of jamming may be detected. A first method of detecting jamming comprises detecting a rise in the output from a correlator chain within a GPS unit. Normally, if a correlator chain is used to detect a GPS signal, the output of one or more correlators will increase in value indicating that a signal has been detected. A rise in the value of all of the correlators in a chain, however, will indicate the possible existence of a jamming signal. If the rise in the value of all of the correlators in a chain is sudden, that too can indicate the presence of a jamming signal. Another artifact of jamming is that it will appear as a clock drift within the GPS circuit. Clock drift can also be due to temperature. If clock drift is present, and there is not a corresponding increase in temperature, this is an indication that jamming may be present. The signal to noise ratio of the spread spectrum signal received by the GPS system may also be used to help detect jamming. Changes in the signal to noise ratio of the GPS signal are normal occurrences within a GPS system as the system and the GPS satellites being received by the system change position. Changes in signal to noise ratio that are sudden, however, can indicate that a jamming signal is present. Coincidence of more than one indication of jamming also makes it more likely that a jamming signal is present.
After jamming has been detected countermeasures may be applied. These countermeasures may include turning the receiver off until the jamming has ceased. Countermeasures to jamming may also include notifying the user that jamming is present and the readings of the GPS are being supressed by the GPS system or that the position determined may no longer may be reliable. Upon detection of a jamming signal dead reckoning methods may also be applied, in lieu of GPS position determination. GPS positioning may then be restored after the jamming has ceased.